Mass spectrometry



Dec. 8, 1953 Filed May 25, 1951 AMPLIFIER 30 c. E. BERRY,

MASS SPECTROMETRY SHIELD 36 3 Sheets-Sheet l INVENTOR. CLIFFORD E. BERRYATTORNEY Dec. 8, 1953' c. E. BERRY 2,662,134

MASS SPECTROMETRY I Filed May 25, 1951 3 Sheets-Sheet 2 FIG. 3.

A SH/ELD CONNECTED J 70 F/LAMENT 1/0- 0 m0 8 SH/ELD AT /2V. W/TH RESPECTk T0 F/LAME/VT Q 90- L] 1 g 80- U 50 T0 REGULATED SHIELD an I F/LAMENTCURRENT- AMPS JNVENTOR,

. CLIFFORD E. BERRY ATTORNE V Dec. 8, 1953 c. E. BERRY MASS SPECTROMETRYFiled May 25, 1951 3 Sheets-Sheet 5 Fl 6. 4. FIG. 5.

E /5 ELECTRON BEAM T 7 if 46 s4 \-/36 ELECTRON BEAM ELECTRON BEAMINVENTOR. CL IFFORD E. BERRY D. C. AMPLIFIER ATTORNEY Patented Dec. 8,1953 MASS SPECTROMETRY Cliflord E. Berry, Altadena, Calif., assignor toConsolidated Engineering Corporation, Pasadena, Calif., a corporation ofCalifornia Application May 25, 1951, Serial No. 228,219

12 Claims. 1

This invention relates to mass spectrometry and mass spectrometers andparticularly to ionization of sample molecules by an electron beam andmethods and apparatus for regulating the intensity of the ionizingelectron beam.

A mass spectrometer is an analytical apparatus which functions to sortand measure ions. Ordinarily it includes an ionization chamber intowhich molecules of the sample to be analyzed are introduced. In theionization chamber the molecules are ionized as by bombardment with astream of electrons, the extent of the ionization being dependent inpart upon the intensity of the electron beam. So-called propelling oraccelcrating electrodes propel the ions from the lOIllzatio'n chamberinto and through an analyzer chamber.

During passage through the analyzer chamber the ions are subjected to atransverse electric or magnetic field, or both, to separate themaccording to their mass-to-charge ratios into a plurality of divergingbeams of ions with each beam being composed of ions of the same specificmass and differing from the specific mass of ions in the other beams.The diverging beams are generally successively focused on an ioncollector by vary ing the potential applied to the acceleratingelectrodes or by varying the intensity of the transverse field in theanalyzer tube. In a fixed focus or monitoring instrument a particularbeam of interest may be continually focused on the collector electrodeand there is no need to scan the ion spectrum by variation of theparameters-affooting the focus thereof. The current produced by thedischarge of an ion beam on the ion collector is a measure of thepartial pressure of those molecules in the sample from which the givenions were derived.

In one type of mass spectrometer, in comparatively general use, ions areformed within an ionization chamber by an electron beam directed throughthe chamber at right angles to the path of the ions and parallel to thedirection of the magnetic field. The ionizing electrons may be developedby an electron emitting filament either in the form of a ribbon or as athin wire, the electrons being collimated as a beam and directed througha slit in a wall of the chamber, transversely across the chamber andthrough a slit in the opposite wall to impinge on an electron targetelectrode.

In co-pending application, Serial No. 52,341, filed October 1, 1948, nowPatent No. 2,611,875, by Harold W. Washburn, consideration was given tothe problem of collimating the electron beam.

In that application the advantages of using a thin wire filament over aribbon filament were explained, and means was disclosed for developing acollimating electrical field to make possible the use of the preferredfilament.

In mass spectrometry it is desirable to hold electron emission from theheated filament of the electron gun at a substantially constant valueindependent of variations in the filament power source and in theemitting properties of the filament. Variation in electron emission andhence in beam intensity introduces a corresponding error factor into theanalysis as a consequence of correlative variation in the degree ofionization. conventionally the desired control of electron emission isaccomplished by electronic feedback means which automatically applies acompensating-adjustment to the power supplied to the electron emittingfilament. Such an electronic system adds appreciably to the cost andcomplexity of the instrument; 'Moreover, any change in power to thefilament results in a corresponding change in filament temperature. Suchresult is undesirable as placing an increased load on the ion sourcetemperature control means.

I have now found that electron emission can be controlled bythe'development of a negative field adjacent the electron filament andon the side thereof opposite the ionization chamber. The invention inone aspect comprises a method of producing ions within a space whichcomprises admitting molecules to be ionized into the space, causingemission of'electrons from a filament adjacent the space, directing theelectrons as a beam through the space, collecting electrons at a targetelectrode spaced from the filament, applying-a negative voltage to ashield electrode adjacent the filament and ion the side thereof oppositethe target electrode and controlling the negative voltage as a functionof the current developed at the target electrode.

Several means are illustrated and describedlfor carrying out theforegoing method, the invention contemplating in addition to the methodan ion source comprising an ionization chamber having a first slit in awall thereof for admitting electrons and a second slit in a wall thereoffor exit of electrons, the combination comprising an electron emittingfilament mounted exteriorly of the ionization chamber in alignment withthe first slit, a target electrode. mounted exteriorly of the ionizationchamber in alignment-with the second slit, a shield electrode mountedadjacent the filament and on the side thereof opposite the first slit,means for impressing a negative voltage on the shield electrode andmeans for varying the magnitude of the negative voltage as a function ofthe current developed by electron discharge at the target electrode.

In the aforementioned co-pending application it was proposedfitoruse ashield electrode in substantially thesame' position as the shieldelectrode in the presently described apparatus and a focusing electrodeintermediate the filament.

and the first slit, and to apply to the shield electrode a negativevoltage and to the focusing electrode a positive voltage with respect:to, theJfila=- ment so as to develop a shaped fieldfforfocusing theelectrons emitted from a thin wire filament through the slit in theionizationchamber. wall. The purpose of focusing the electrons by meansof a shaped field was to avoid the necessity off The invention will-be.more clearly understood from the following detailed-description taken inconjunction with the-accompanying, drawings in which:

Fig. 1 is a-Ldiagrammatic view" of a conventional 180- mass spectrometemFig. 2 is an, enlarged section taken on the line 2'---2 of Fig. 1;. c

Fig. 3.-is a graph showingxthe effect on anode or target electrode*currentofavarious connections of theshield electrode; j

Fig; 4" is a: circuit diagram showing; one. means of connectingthe'shieltl electrode to regulate filament, emission in. accordancewithrthe: invention;

Fig:v 5 is; a; circuit. diagram of: an alternative means foraccomplishingthis regulation;

Fig. 6 is a circuit diagram of another means of accomplishing; this:regula'tidm; and' Fig: 7 is a diagraniiiofc amalternative form; ofregulator circuit? including: amplification meansz.

Im the; drawingsthe'. inventiom iSTShOWIl'. irr. as sociatiorr with: a.180"" massi spectrometer; of' the type wherein propelling electrodesiare: provided i111 theI-ioir source. 'Aswill betapparentt from thefollowing; description; me invention? is equally adapted: toincorporationzin substazrrtially' any mass: spectrometer: whereimtiie.sample: to be analyzed isionized by meansorzanielectrombeann It: isalso. apparent that the: invention. is: not limitedito-thefieldcoffmassspectrometry but: will find: application in. ion source. iniwhich:uniformity of an ionizing-:eledtrmr beautis sought-i.

- InaFigsr. L and; thermis shcwnialmass-spectrometerilnhavingaaniorcsourcer than analyzer tube t2; an: ion: collector. [$.1a112disposdzwithin anenvelop e: I 4 whiclris keptrat lowpressuresi'dur ing' the.-operation-0f. the; instrument. A trans:- verse magneticifieldisxestahlishediin1 the; analyzer tub e by conventionalmagnet: means:(not shown).- mounted exteriorlyr ofi the. envelope; The analyzer tubel2: provided atzitsend: adjacent the ion; collecton BJWith: an; exit:slit 55 through which; thezionibeamsareafomsed?omthacollectmi electrode;I3.;-

The; spectrometer is; with: a. pumping system or envelope exhaust line[5 which may be connected with a mercury diffusion pump, molecular pump,or any appropriate evacuating system (not shown). The analyzer tube 12may be provided with ports I! by means of which the ion source and theanalyzer tube are evacuated through the envelope. Alternatively theenvelope may be; omitted by making the analyzer tube gastight and byattaching the evacuating system directly to the analyzer tube by meanswhich are well known. An inlet line l8 provides means for introducing asample to be analyzed.

An electron gun 20, in accordance with the invention, is mountedfadjacent to or as part of the ion sourceand cooperates with an electrontarget 1 2iI-to discharge-a beam 19 of electrons through a first.v slit22,in.a.wall of the ion source adjacent the gun 2D and a second slit 23in an opposite wall of the source adjacent the target electrode 2|.

Bropelling. electrodes 25 and accelerating electrodes26g21 are disposedwithin the ion source to expel the ions formed therein into the analyzerchamberand at a sufficient velocity to carry them through theanalyzerchamber. Representation of this particular ion source is forillustrative purposes only, therebeing many modifications thereofequally applicable/to this invention.

Asabove described, the ions propelled from the source are formed in-theanalyzer tube under the influence of the transverse magnetic field intodiverging beams of ionsof given specific mass. Thesediverging beams arevsuccessively focused on'the ion collector l3 through the exit slit l5 byvarying the-potentials applied to the accelerating electrodes 26, 2.1.The ion collector I3 is linked with an amplification and sensingcircuitvby a lead 29 sealed through a wall of the envelope l3andconnectedexteriorly to an amplifier 30 which is,1in turn; linkedtosensing means (not shown) suchas arrecorder or the like.

Acceleratingelectrodes 26, 21 are connected throughleads 32,33'toaconventional voltage supply circuit (not shown). In a similarmanner tliepropelling electrode 25 may be and generally is connectedinto the-same voltage supply circuit.

The electron-gun shown: inFig. 1 comprises a filament 34' preferably ofcircular. cross section and. disposed adjacentthe first electron slit22. A shield electrode 3.6 is disposed adjacent the filament 3.4 onthe.side opposite the slit 22. The shield: electrode 36. is=shown :asarcuate insection. However; there isno requirement in the presentinvention that: this' electrode assume any particular shape:

The filament34-,-shield electrode 36 and electronitar'getrzll areinterconnected in accordance with the invention in onet-of a-number ofcircuits whereby the shield 36$is maintained at a negative potentialwith respect to the filament. 34 and the magnitude:ofthenegative voltageapplied to the shield at isdetermined as-a function of a biasvoltage'source and 'of.the1current collected at the target: '21::Various circuits for accomplishing thisz'purposeareiillustrated in Figs.4,5, 6 andfl, hereinafter discussed 2 Reference to; Fig. 3 will giVeTaclear understanding'of the objectives of the. invention. Fig. 2shows-"the variation in anode current. (abscissa) as-a functionoffilamentcurrent (ordinate) unthe, relationship. between. filamentcurrentand anode current with the shield held at a constant potential of minus12 volts with respect to the filament. Curve C shows the relationship offilament current and anode current with the shield regulated inaccordance with the present invention. The slope of the plateau in curveC is such that a 3.3% change in filament current causes a 1% change inanode current.

It is plain from curves A and B of Fig. 3 that for a given filamentcurrent, the anode current decreases rapidly as the shield is made morenegative with respect to the filament. It is this characteristic thatenables the shield electrode to be used as a regulating element tocontrol the anode current.

One circuit for energizing the electron gun and controlling the voltageapplied to the shield electrode is shown in Fig. 4. In Fig. 4 and in thesucceeding figures, the shield, filament, and

target electrodes are given the numbers 35, 34, 2|,

respectively, to conform these figures with Fig. 1. The target electrode2| is connected directly to the shield 36 through a bias battery Allwhich delivers a negative voltage to the shield 36. A resistor 42 andbatteries 54, 36 are connected between the target 2| and the filament 34with the ion source connected between the batteries 44 and 46.

The operation of the circuit of Fig. 4 is as follows: The filament isoperated so as to supply an excess of electrons, and the filament shieldis 4 biased negative with respect to the filament to a suitableoperating point by th battery 40. The anode current flows through theresistor 42 which is of comparatively high resistance, variations in theanode current causing corresponding variations in the voltage across theresistor. These voltage changes are directly coupled to the shieldelectrode 36 and are in such a direction that an increase in anodecurrent causes an increase in the negative shield voltage. The effect ofan increase in the negative shield voltage is to decrease the potentialof the space between the filament and electrode I, which has the effectof decreasing the current to the target 2|. Thus any tendency for thetarget current to change is counteracted by the action of the shield.

A somewhat different circuit arrangement is shown in Fig, in whichtarget electrode 2| is connected directly through batteries 44 and 40 tothe shield electrode 35. The resistor 52 and the filament battery 46 areconnected to the anode 2| intermediate the batteries llland 44, and theion source H is connected intermediate resistor 42 and battery 56.

The operation of the circuit of Fig. 5 is similar to that of Fig. 3, theonly differenc being that the particular arrangement reduces the totalbattery voltage required.

Fig. 6 shows yet another arrangement of the same components includingthe three batteries 40. 44 and 46 and the resistor 42. The circuit ofFig. 6 again has an advantage of minimizing the battery voltagerequirements. As an example of the operating characteristics of thesystem of Fig. 6, battery 40 may be a 22.5 volt battery, battery 44 a2'70 volt battery and battery 46 a 90 volt battery. Resistor 42 has avalue of 1.75 megohms, so that with a current of 58 ra, the nominalshieldto-filament voltage is minus 12 volts.

Th circuit of Fig. 7 differs from that of Fig. 1 in the inclusion of aD, C. amplifier in the lead connecting target electrode 2| with theshield electrode 36. An amplifier may be included in any of theillustrated circuits to increase the sensitivity of the system tochanges in collected ponents may be altered, the specific values givenfor voltage sources and resistors being by way of example only, and inno way limiting. In the same respect illustration of batteries in theseveral described circuits is intended to be representative of any D. C.voltage source, it being ob.- vious that there is no limitation to thederivation of power from any particular type of source.

I claim:

1. In an ion source comprising an ionization chamber, the combinationcomprising an electron emitting filament, a target electrode forcollecting electrons, a shield electrode mounted adjacent the filamenton the side thereof opposite the target electrode, means for directingthe electrons through a portion of the ionization chamber to the targetelectrode, means for impressing a negative voltage on the shieldelectrode, and

means for varying the magnitude of the negative voltage as a function ofthe current developed by electron discharge at the target electrode.

2. In an ion source comprising an ionization chamber having a first slitin a wall thereof for admitting electrons and a second slit in a wallthereof for exit of electrons, the combination comprising an electronemitting filament mounted exteriorly of the ionization chamber inalignment with the first slit, a target electrode mounted exteriorly ofthe ionization chamber in alignment with the second slit, a shieldelectrode mounted adjacent the filament on the side thereof opposite thefirst slit, means for impressing a negative voltage on the shieldelectrode, and means for varying the magnitude of the negative voltageas a function of the current developed by electron discharge at thetarget electrode.

3. In an ion source comprising an ionization chamber having a first slitin a Wall thereof for admitting electrons and a second slit in a wallthereof for exit of electrons, the combination comprising an electronemitting filament mounted exteriorly of the ionization chamber inalignment with the first slit, a target electrode mounted ex teriorly ofthe ionization chamber in alignment with the second slit, a shieldelectrode mounted adjacent the filament on the side thereof opposite thefirst slit, means for holding the shield electrode at a negativepotential with respect to the filament, and means for varying the shieldelectrode to filament potential as a function of the current developedby electron discharg at the target electrode.

4. In an ion source comprising an ionization chamber having a first slitin a wall thereof for admitting electrons and a second slit in a wallthereof for exit of electrons, the combination comprising an electronemitting filament mounted exteriorly of the ionization chamber inalignment with the first slit, a target electrode mounted exteriorly ofthe ionization chamber in alignment with the second slit, a shieldelectrode mounted adjacent the filament on the side thereof opposite thefirst slit, a first source of bias voltage connected to maintain theshield electrode at a negative potential with respect to the filament,and a second source of bias voltage operable to vary the shieldelectrode to filament potential as; a function of the currentdevelopedby ment with the first slit, a target electrode mountedexteriorly of the ionization chamber in alignment with the second slit,a shield electrode mounted adjacent the filament on the side thereofopposite the first slit, a first source of bias voltage connected tomaintain the shield electrode at a negative potential with respect tothe filament, means for developing a voltage responsive to and as afunction of the current developed by electron discharge at the targetelectrode, means for amplifying this voltage, and means for applying theamplified voltage to vary the shield electrode to filament potential.

6. In an ion source including an ionization chamber, the combinationcomprising an electron emitting filament, a target electrode forcollecting electrons, means for directing electrons from the filamentthrough a portion of the ionization chamber to the target electrode,means developing a potential between the filament and ionization chamberto direct electrons emitted from the filament toward the ionizationchamber, and means for varying this potential as a function of thecurrent developed by electron discharge at the target electrode.

'7. In an ion source including an ionization chamber having a first slitin a Wall thereof for admitting electrons and a second slit in a wallthereof for exit of electrons, the combination comprising an electronemitting filament mounted exteriorly of the ionization chamber inalignment with the second slit, means developing a potential between thefilament and ionization chamber to direct electrons toward the firstslit, and means for varying this potential as a function of the currentdeveloped by electron discharge at the target electrode.

8. In an ion source including an ionization chamber having a first slitin a wall thereof for admitting electrons and a second slit in a wallthereof for exit of electrons, the combination comprising an electronemitting filament mounted exteriorly of the ionization chamber inalignment with the first slit, a target electrode mounted exteriorly ofthe ionization chamber in alignment with the second slit, meansdeveloping an electronfocusing field in the region of the filament tofocus electrons through the first slit, and means for varying thefocusing characteristics of the field as a function of the currentdeveloped by electron discharge at the target electrode.

9. In an ion source including an ionization chamber, the combinationcomprising an elec; tron emitting filament, a target electrode forcollecting electrons, means for directing electrons from the filamentthrough a portion of the ionization chamber to the target electrode todevelop a current at the target electrode proportional to the number ofelectrons discharging thereon, means developing a potential between thefilament and ionization chamber, and means for varying this potentialresponsive tovariations in the current developed by electron dischargeat the target electrode whereby this current is maintained substantiallyconstant in magnitude.

10. Apparatus for producing ions within a space which comprises meansfor introducing molecules to the space, electron producing means, meansdeveloping an electrical field for directing electrons from the electronproducing means through the space, means for collecting electrons afterpassage thereof through the space and developing a current proportionalin magnitude to the number of electrons collected, and means for varyingthe magnitude of the field responsive to variations in the magnitude ofthe current so as to vary the number of electrons directed through thespace to hold the current at a substantially constant value.

11. Apparatus for producing ions within a space which comprises meansfor introducing molecules to be ionized into the space, electronproducing means, means developing an electrical field for directingelectrons from the electron producing means through the space, meansdeveloping a negative electrical field in the region of electronproduction, means for collecting electrons after passage through thespace, and means controlling the negative field as a function of thecurrent developed by collection of the electrons.

12. Apparatus for producing ions within a space which comprises meansfor introducing molecules to be ionized to the space, an electronemitting filament disposed adjacent the space, means directing electronsfrom the filament through the space, means for collecting electronsafter passage through the space, an electrode disposed adjacent thefilament and on the side thereof op,- posite the space, meansmaintaining said electrode at a negative potential with respect to thefilament, means controlling the magnitude of the negative potential as afunction of the current developed at the electron collecting meansresponsive to discharge of electrons thereon. V

CLIFFORD E. BERRY.

References Cited in the file of this patent UNITED STATES PATENTS NumberName I Date 2,373,151 Taylor Apr. 10, 1945 2,457,530 Coggeshall et alDec. 28, 1948

